This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present techniques, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
The present disclosure relates generally to methods and systems of model-based acoustic measurements for a perforated casing in a wellbore. In particular, the present disclosure relates to methods and systems of model-based cement bond log (CBL) measurements in a perforated casing for a plug and abandon (P&A) activity.
When an oil producing well is no longer profitable to oil companies, they take decisions to abandon the well after plugging the well, either temporarily or permanently depending on the economy and provision of redevelopment of the well. Permanent abandonment is called plug and abandon (P&A), as described in “Offshore Permanent Abandonment”, Oilfield Review Spring 2012, Schlumberger. The P&A is costly operations in particularly in deep-sea market. Oil companies are liable to any catastrophic consequences in well integrity issues caused by poorly abandoned well even long time after abandonment, a reliable P&A process with its quality control is substantially important for both safety and environmental reasons.
One of the key and challenging processes of P&A operation is to set a cement plug in tubing or casing to ensure zonal isolation or no cross-flow behind the pipes. The method of setting the cement plug is to mil the casing (or tubing) before injecting cement in the depth interval where zonal isolation is intended. This milling operation requires highly weighted viscous mud to lift swarf (or steel debris) and drilled cement that may prevent multidirectional seal. The swarf-laden mud exceeds density that rock in milled section can withstands and leads to rock failure. Alternative to milling method, there is a process known as perforate, wash and cement (PWC) as described in “Offshore Permanent Abandonment”, Oilfield Review Spring 2012. The process utilizes perforation guns to make holes, cleaning or washing out perforation debris before squeezing cement. Although the operation does not have risk of rock failure, there are uncertainties in status of perforated casing, in particularly the success of cement squeeze in term of hydrocarbon isolation.
Acoustic cement evaluation techniques such as an ultrasonic cement mapping, sonic cement bond log (CBL) and CBL-variable density Log (CBL-VDL) are present to be applied to an oil well for cement evaluation For example, the sonic Cement Bond Log (CBL) is developed in 1960s to evaluate casing/borehole cementing quality of oil wells as described in G. H. Pardue et al., “Cement Bond Log—A Study of Cement and Casing Variables”, 1963, Journal of Petroleum Engineering, SPE 453 which is incorporated herein in its entirety by reference thereto. The CBL-VDL technique is described in H. D. Brown et al., “New Development in Sonic Wave Train Display and Analysis in Cased Holes”, SPWLA Eleventh Annual Logging Symposium, May 3-6, 1970 which is also incorporated herein in its entirety by reference thereto.
However, these cement evaluation techniques are developed for the standard steel casings. Logging operation may be available in P&A wells, however, a problem raises in interpreting the data. Ultrasonic cement mapping provides localized (30-150 mm) cement quality behind casing, however, when casings are heavily perforated, more than half of casing surface is lost and there are uncertainties of ultrasonic response due to presence of perforation. The CBL can be used for measurements of cement quality in larger scale (60 mm-1000 mm) using casing extensional mode, however, there is no proper database to perform quantitative cement evaluation in a perforated casing.
As will become apparent from the following description and discussion, the present disclosure provides improved methods of sealing process in downhole applications.